The invention relates, generally, to ink jet printers and, more particularly, to an improved printhead for such printers.
A typical printhead, shown in FIG. 1, consists of an apparatus 2 for generating ink drops that are electrically charged by a charging electrode 4. The charging electrode must be dimensionally accurate, have sufficient electrical conductivity to charge a drop, be able to shield the ink stream from high voltage generated electrical fields and noise and must be corrosion resistant. The known prior art charging electrodes typically include a machined or formed metal electrodes consisting of machined metal plates or machined metal cylinders. The electrodes are typically fixed to an insulated member such as the housing, support arms, or printed circuit boards. The electrodes are fixed to the insulated member by a variety of fasteners such as screws, machined supports or soldering. Examples of such prior art charging electrodes are the Hitachi GXII-S and Videojet EXCEL, Domino Solo 5, Hueft Printer, Willett 3850 and Image Series 7. While such charging electrodes perform satisfactorily they are complex and costly to manufacture because of the number of parts and the materials involved. Moreover, because these electrodes are made from corrosion resistant metals they require expensive fabrication processes.
The charged drops are then selectively deflected by an electric deflection field created between plates 6 and 8 to create a desired printed image on a substrate. The drops that are not deflected onto the substrate are retrieved in an ink catcher 10 and recirculated.
These components are carried by a chassis 12 that is connected to a duct 14 carrying wires for the electrical signals and an ink supply line. A nozzle cover 11 and printhead cover 13 surround chassis 12 to protect the internal components. The chassis 12 is typically constructed of conductive metal in order to perform three separate functions. First, ink catcher 10 is connected to a phase sensing circuit that determines the proper amplifier phasing for the charging electrode. The sensing circuit is extremely sensitive and must be able to detect electrical currents on the order of nanoamps. Thus the sensing circuit must be shielded from electrical noise from the nozzle, charging tunnel and other components. The chassis, because it is electrically conductive, is grounded to provide such shielding.
Second, when the ink drops contact either the substrate or ink catcher, a microscopic charged fog is created due to ink spatter. Over time, this fog will build up on the printhead components to create errant electric fields which can misdirect the drops. By using a metal chassis and connecting it to ground, the charge from the fog is discharged and the errant electric fields are minimized.
Third, the chassis, because it is electrically conductive and connected to ground, can form a part of one of the deflection plates.
While a metal chassis performs these functions well, it is expensive to fabricate because of the relatively complex structure required. In an attempt to eliminate the use of the metal chassis, non-conductive plastic chassis have been developed. One such example of a plastic printhead chassis is the Willett 3800 which consists of a two-piece, hinged, insulating plastic chassis in which the nozzle, charge tunnel, charge sensor, ink catcher and high voltage deflection plates are mounted. The two-piece chassis is of a clam shell arrangement which forms an enclosure for these functional elements. The charge tunnel and one of the deflection plates are mounted to the pivoting portion of the chassis. The deflection plates are both mounted on insulating plastic blocks which, in turn, are mounted in the chassis. The charge tunnel consists of insulated parallel plates mounted to a metal fixture which is mounted to the plastic chassis. The structure is a monocoque with the two piece chassis serving as the outer cover.
The two piece chassis described above is mounted to the front of a metal box portion of the printhead which contains wiring connections. All wiring connections in the chassis are rigidly attached to their corresponding functional elements (i.e. the charge tunnel and deflection plates) and potted in place. No provision is made for ink mist discharge other than the wide spacing of the elements in the printhead. Moreover, the absence of a grounded cover renders this design susceptible to environmental electrical noise.
Another prior art plastic chassis is the Image Series 7 which consists of an elongated box having the nozzle, charging plates and deflection plates mounted to the outer surfaces thereof. Portions of these components extend through this surface into the interior of the box. All electrical connections to these components are made on the interior of the chassis by soldering shielded cables directly to the corresponding functional element. The plastic chassis is, in effect, an insulating mounting arm for the functional components. Noise sensitive connections are surrounded by separate metal cans to minimize noise interference. The front of the chassis is covered by a metal plate to help shield signals from external noise sources and to discharge externally accumulated ink mist. There is no provision, however, for discharging the ink mist on the internal surface of the chassis.
The use of the insulating plastic chassis of the prior art requires complex electrical connections and wiring details. Additionally, the discharge of the ink mist and noise protection can only be accomplished, if at all, by means of ancillary components. Finally, such printheads require two separate deflection plates because the insulated plastic chassis cannot be used for one of the plates.
Thus, an improved printhead chassis is desired that is less expensive than prior art chassis yet is capable of performing the functions of a metal chassis.